In a water heater, a water supply pipe and a hot water outlet pipe are coupled to a heat exchanger, which is heated with a burner. When a faucet is opened to pass water through inside an apparatus, a controller (an operation control means) that detects the passing water causes the burner to burn to heat the water that passes through the heat exchanger. Then, hot water is output from the hot water outlet pipe. Among such water heaters, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-57845 (JP-A-2008-57845), there is known a water heater that includes a passing water quantity control means, such as a water servo, that controls a passing water quantity in the heat exchanger in the water supply pipe. The controller performs a combustion control of the burner and an operational control of the passing water quantity control means to perform an output hot water temperature control. The output hot water temperature control causes a detected temperature (output hot water temperature) obtained from a temperature detection means, such as a thermistor, disposed in the hot water outlet pipe to match a set temperature.
However, in the water heater of JP-A-2008-57845, at a start of a hot water supply, the passing water quantity controlled by the passing water quantity control means is set to a predetermined water quantity. Therefore, in the case of what is called a cold start, the output hot water temperature takes time to reach the set temperature and a consumption quantity of water and fuel gas during that period increases, thereby leading to a loss. The cold start is when a temperature of inflow water is low when a power supply is first turned on to start an operation after the water heater is installed or when the operation is started after a lapse of long time since the last hot water supply.
Therefore, the applicant has provided the following disclosure in Japanese Unexamined Patent Application Publication No. 2010-117053 (JP-A-2010-117053). The operation control means compares the detected temperature obtained from the temperature detection means with the set temperature at the start of the hot water supply. When the detected temperature is lower than the set temperature by a predetermined amount, the output hot water temperature control is executed by configuring the passing water quantity control means to have a passing water quantity that is further restricted compared with the predetermined water quantity. Thus, the reach time to the set temperature is reduced even in the case of the cold start, thereby ensuring conserved water and gas.
In the passing water control in JP-A-2010-117053, after the output hot water temperature matches the set temperature, the restriction of the passing water quantity needs to be gradually released to return the passing water quantity back to the predetermined water quantity. However, there is a case where the burner is constituted of a plurality of stages of units (burner group) that are divided into each of a plurality of burners, each of which includes mutually different numbers of burners, and performs a switching control of combustion stages by selecting the unit to burn. In such case, for switching of the combustion stages, a control is performed such that a gas input is once decreased to transfer a fire to a neighboring unit and then the gas input is increased in order to smoothly transfer the fire. Therefore, in spite of performing a control to increase the output hot water temperature at the start of the hot water supply, the control to decrease the gas input is temporarily performed due to switching of the combustion stages. As a result, the output hot water temperature does not linearly increase proportionately to an increase of the passing water quantity, and an undershoot possibly occurs, which fluctuates reacting to the increase and decrease of the gas input.
The following describes what is mentioned above specifically. First, FIG. 4 is a graph showing a restriction control of the passing water quantity at the start of the hot water supply and change of the output hot water temperature. A dotted line indicates the passing water quantity and a solid line indicates the output hot water temperature. The restriction control here is the following control. When an ignition is started at t1, the passing water quantity is restricted from the predetermined water quantity until t2. After the restricted passing water quantity is maintained until t3, the restriction is gradually released to return the passing water quantity back to the predetermined water quantity at t4. Accordingly, it is ideal that, with this restriction control, the output hot water temperature linearly increases and stabilizes at the set temperature as indicated by a two-dot chain line.
Meanwhile, FIGS. 5A and 5B illustrate a switching control in the case where there are three stages (three units) of burners. As illustrated in FIG. 5A, at the start of the hot water supply (t1), a burner in the second stage is used at an intermediate input. In association with the restriction of the passing water quantity, after the gas input is restricted to a lower limit input as indicated by a solid line arrow, the gas input is switched to a lower limit input of the first stage as indicated by a dotted arrow. After the gas input is increased from that point to cause a fire to transfer, further in association with the restriction of the passing water quantity, the input is restricted as indicated by the solid line arrow. In the example of FIGS. 5A and 5B, the input is maintained at points indicated by black points in the solid line arrow when the passing water quantity is at the lower limit (between t2 and t3).
Then, as illustrated in FIG. 5B, when the restriction of the passing water quantity is released from t3, after the gas input is increased to an upper limit input of the first stage as indicated by the solid line arrow, the gas input is restricted to the intermediate input as indicated by the dotted arrow. The fire is transferred in a state where the gas input is slightly restricted from the intermediate input of the second stage. Next, as indicated by the solid line arrow, after the gas input is increased to the upper limit input of the second stage, the gas input is restricted to the intermediate input as indicated by the dotted arrow. The fire is transferred in a state where the gas input is slightly restricted from the intermediate input of the third stage and the input is increased until t4 as indicated by the solid line arrow.
Thus, since switching of the combustion stages is performed twice between t3 and t4 in which the restriction of the passing water quantity is released, the output hot water temperature does not linearly increase like the two-dot chain line illustrated in FIG. 4 but to fluctuate in a portion T1. Therefore, the undershoot occurs.